Hi @kpfrank
The part number is ''486665-01'' (found in this [https://www.ereplacementparts.com/black-and-decker-rt550b-type-wizard-rotary-tool-parts-c-4167_4332_12014.html|link] in the Discussion below the parts for a RT550B type 2) but there are many [https://www.ereplacementparts.com/search_result.php?q=rt550|RT550 models] so I’m not sure if it will fit all of the models.
I’ve found two suppliers by searching online for the part number ''only,'' that may help if they’re located near to you that is. [https://www.sparetoolparts.co.uk/black-decker-rt550ka-spring-486665-01-spare-part-type-1.html|supplier 1], [https://www.solenttools.co.uk/parts-by-brand/black-and-decker/rt550.html|supplier 2]
All the other results show out of stock, no longer available due to manufacturer not making it anymore.